332
J. A. Delgado and C. Godard
When stirring rates higher than 1500 rpm were tested, no gas–liquid (G–L) masstransfer limitations were observed while the alkenol selectivity increased with the
stirring rate. In contrast, minor variations of the hydrogen liquid–solid (L–S) were
registered, while the substrate L–S mass transfer turned relevant under high hydrogen
concentration and high conversions.
Less intuitive parameters such as the heating method have also revealed to
affect the catalysts reactivity in this reaction. For instance, Wu et al. reported the
effect of using microwave irradiation as heating method in the aqueous phase semihydrogenation of 2-butyne-1,4-diol catalysed by a Pd/Bohemite catalyst [171]. The
presented catalytic system heated by microwave irradiation evidenced enhanced
conversion when compared to normal heating while the selectivity was maintained.
The same authors also investigated the impact of ultrasound in the semi-semihydrogenation of phenylacetylenes employing the mentioned catalyst [172]. Interestingly, higher alkene selectivity was observed by the application of ultrasound, a
phenomena ascribed to the accelerated desorption of the alkene product from the
metal surface thus preventing the over-reduction reaction.
Very recently, Fukazawa et al. reported an unconventional approach for the electrocatalytic hydrogenation of alkynes in a proton-exchange membrane reactor (PEM)
[173]. The alkene selectivity resulted sensitive to the catalyst material and the cathode
potential. From the tested catalysts (Pd, Pt and Rh based), Pd/carbon was the more
selective one, with the highest current efficiencies at relatively low electrolysis potentials (too negative potentials favoured the over-hydrogenation reaction). Although
quite demanding in terms of the technical infrastructure required for the electrocatalytic reaction, this approach enables the hydrogenation employing only water as
hydrogen source, and might represent an alternative when pressurized hydrogen or
reactors are not available.
10.6 Conclusions
The preparation of well-defined nanostructured materials is nowadays accessible
thanks to the cumulative efforts in the area of materials and catalysis. The availability of such tools, combined with the understanding of the structural and molecular parameters that rule the reactivity in the semi-hydrogenation of alkynes, permits
the rationalization of highly structured nanostructures tailor-made for the specific
reaction of application.
From this, not exhaustive but comprehensive review of the relevant literature,
it can be summarized that most of the strategies evaluated to enhance the alkene
selectivity can be classified in one of the following three categories (Fig. 10.17).
First, we can distinguish those assigned as “surrounding strategy” that includes
the use of unreactive materials that wrap, surround or simply accompany the active
phase (e.g. the solid support). Reaction conditions are included in this category as
global parameters that can condition energy states of the whole catalyst, availability
of hydrogen (and reactants for gas phase reaction) or diffusional phenomena.
J. A. Delgado and C. Godard
When stirring rates higher than 1500 rpm were tested, no gas–liquid (G–L) masstransfer limitations were observed while the alkenol selectivity increased with the
stirring rate. In contrast, minor variations of the hydrogen liquid–solid (L–S) were
registered, while the substrate L–S mass transfer turned relevant under high hydrogen
concentration and high conversions.
Less intuitive parameters such as the heating method have also revealed to
affect the catalysts reactivity in this reaction. For instance, Wu et al. reported the
effect of using microwave irradiation as heating method in the aqueous phase semihydrogenation of 2-butyne-1,4-diol catalysed by a Pd/Bohemite catalyst [171]. The
presented catalytic system heated by microwave irradiation evidenced enhanced
conversion when compared to normal heating while the selectivity was maintained.
The same authors also investigated the impact of ultrasound in the semi-semihydrogenation of phenylacetylenes employing the mentioned catalyst [172]. Interestingly, higher alkene selectivity was observed by the application of ultrasound, a
phenomena ascribed to the accelerated desorption of the alkene product from the
metal surface thus preventing the over-reduction reaction.
Very recently, Fukazawa et al. reported an unconventional approach for the electrocatalytic hydrogenation of alkynes in a proton-exchange membrane reactor (PEM)
[173]. The alkene selectivity resulted sensitive to the catalyst material and the cathode
potential. From the tested catalysts (Pd, Pt and Rh based), Pd/carbon was the more
selective one, with the highest current efficiencies at relatively low electrolysis potentials (too negative potentials favoured the over-hydrogenation reaction). Although
quite demanding in terms of the technical infrastructure required for the electrocatalytic reaction, this approach enables the hydrogenation employing only water as
hydrogen source, and might represent an alternative when pressurized hydrogen or
reactors are not available.
10.6 Conclusions
The preparation of well-defined nanostructured materials is nowadays accessible
thanks to the cumulative efforts in the area of materials and catalysis. The availability of such tools, combined with the understanding of the structural and molecular parameters that rule the reactivity in the semi-hydrogenation of alkynes, permits
the rationalization of highly structured nanostructures tailor-made for the specific
reaction of application.
From this, not exhaustive but comprehensive review of the relevant literature,
it can be summarized that most of the strategies evaluated to enhance the alkene
selectivity can be classified in one of the following three categories (Fig. 10.17).
First, we can distinguish those assigned as “surrounding strategy” that includes
the use of unreactive materials that wrap, surround or simply accompany the active
phase (e.g. the solid support). Reaction conditions are included in this category as
global parameters that can condition energy states of the whole catalyst, availability
of hydrogen (and reactants for gas phase reaction) or diffusional phenomena.